On Sunday, a stalled frontal boundary over southern New England will lift northward as a warm front. This will happen as a surface low tracks to the northwest of the region. This will put most of New England into the system's warm sector with heat and humidity surging once again. This setup will be favorable for severe thunderstorm development across the region.
Rounds of showers and storms will continue to move through New England this morning. These showers will be most likely from Lake Winnipesaukee points north with more isolated activity in central and southern New England until the afternoon and evening. This morning activity may hinder severe weather development for areas farther north if the atmosphere is not able to re-energize by the time the afternoon activity begins to bubble up.
After the morning round, there will likely be a period of much more isolated activity before the main activity begins to erupt in the afternoon and evening. The main time frame for severe storms will likely be from 1pm to 6pm for western areas and 4pm to 9pm for areas farther east. Timing has continued to trend toward a later start time. Storms will likely be more scattered in nature than in a solid line. A broken line of storms may form.
GFS showing potential weather around mid to late afternoon:
The greatest chance for severe weather will be southern Vermont, New Hampshire and western Massachusetts. With that said, all of New England outside of eastern Maine will have a chance to see a stronger storm. The areas mentioned before will likely see the most favorable timing of storms combined with the strongest severe weather ingredients. It's starting to look like southern New England may not see as much severe weather activity as Vermont and New Hampshire.
The greatest threat from severe storms on Sunday will be potentially damaging wind gusts. This will come as an unusually strong (by summer standards) low level jet crosses the region. This jet will aid in pushing the warm front to the north. The jet will also create gusty southwest winds outside of storms. Strong to severe storms will only enhance these already gusty winds.
The tornado threat is elevated by New England standards. The Storm Prediction Center has put southern Vermont and much of New Hampshire in the 10% zone for tornadoes. This is only the third 10% tornado risk issued for this area since 2010. Shear profiles over this area are supportive of discrete rotating supercells.
Storm Prediction Center tornado outlook:
The extent and placement of the threats above will be contingent on the northward extent of the warm front. The most favorable environment for severe weather will be in the warm sector, which will be near and south of the warm front. The timing of the front's northward movement will be critical in storm placement. Areas north of the front will see much more stable air and have a much more difficult time seeing storms fire.
1st image below: weather map showing the front's location as of Sunday morning. 2nd image below: Forecast front position by Sunday afternoon, showing the northward advancement:
Large hail will likely be the most limited threat of the three main severe threats (wind and tornadoes being the other two). Lapse rates (the rate of temperature decline with altitude) will be mediocre for hail development and the freezing level will be around 14,000 to 15,000 feet, which will make it difficult for large hail to reach the ground. With that said, strong updrafts would allow for larger hail, but this threat should be pretty isolated.
As far as the four main severe ingredients go, deep layer shear is expected to be around 40-50 knots, which is sufficient for supercell development. Lift will be provided by the system’s fronts. Moisture will be ample with high humidity levels in the warm sector. Dew points will likely rise into the upper 60s to low 70s.
The big question, as always for New England, will be the amount of instability. The heat and humidity of the warm sector will provide instability, but if it remains cloudy for the entire day, CAPE values will not be able to climb. The southwest flow should help clear clouds out, but clouds will try to be stubborn and stick around. That will be the battle when it comes to severe weather. CAPE values are expected to climb into the 15,00-2,000 range.
Forecast (surface based) CAPE values from HRRR this afternoon. You can see values sharply drop off in northern New Hampshire, likely due to the warm front positioning:
With all of that said, if clouds linger for much of the day, instability will not build as high as expected and therefore reduce the threat. The morning activity may also rob the atmosphere of energy for the afternoon storms. If the atmosphere can’t rebuild this energy (mainly due to a cloudy sky), afternoon activity will not be as widespread.
If the morning activity lingers, this would also impede activity later in the day. This is not a slam dunk forecast for severe weather, but conditions certainly favor a rather high end (by New England standards) severe weather day.
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